This application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/JP99/03650 (not published in English) filed Jul. 6, 1999.
1. Technical Field
This invention relates to a microwave discharge apparatus for creating and keeping a plasma with efficiently absorbing microwave and not generating much reflected wave, and efficiently cooling the discharge vessel made of dielectric substance.
2. Background Art
Surface treatments such as etching, Plasma Chemical Vapor Deposition (PCVD) and heat treatment, has already been applied to various industrial fields. Particularly, Plasma Downstream Treatment with setting a sample in the down stream of plasma causes little damage on the sample and is expected for various applications and developments mainly in the field of semiconductor in the future.
The most important role of plasma in Plasma Down Stream Treatment is to efficiently generate active particles for use in the process. A microwave on 2.45 GHz band is often selected to excite plasma, because plasma-generating apparatuses using the waveband can be produced at low cost and the microwave with the band range enables more efficiently dissociating gas molecules than RF.
The most typical type of practical microwave plasma-generating apparatuses has a structure that a cylindrical dielectric tube, often made of quartz, is provided along an axis vertical to a rectangular-waveguide wall normal to the electric field direction of the microwave, or the H plane wall, through the center of the wall.
A dielectric tube is always exposed to an incident wave in the electric-discharge structure if an unreflective termination is formed at a point reached by the microwave passing through the tube, on the opposite side to the magnetron. However, a terminal face is generally formed with a plunger in order to reflect a wave to generate a standing wave and the dielectric tube is located at the maximum point in the electric field. The structure is applied to generate plasma easily in the stronger electric field than the one created just by the incident wave and to convert the energy efficiently from the microwave into plasma.
However, when plasma is generated, the impedance in the dielectric tube changes considerably, from the previous one almost same as the one under vacuum, and then the electric field is disturbed around the dielectric tube and has the maximum value at another point.
The proper gas composition and pressure etc. should be selected to apply to a requested plasma treatment. The impedance of the plasma depends on the factors such as gas composition and pressure etc. Therefore, the phase of standing wave at the dielectric tube should be properly tuned in the process with, for example, a plunger. Consequently, the structure is not practically useful. Further, the dielectric-tube cannot include many electric peaks of the microwave due to the structure itself. The energy is converted into plasma at low rate. The structure suffers much loss in the thermal conversion.
Generally, an isolator and a matching device are provided between the microwave emitting part and the electric-discharge load to deal with the reflected standing wave. The structure costs much because of the attachment.
The plasma-generating apparatus, which has a cylindrical dielectric tube penetrated obliquely from the center of the H plane wall or located parallel to the direction in which the microwave travels at the cross-sectional center of the waveguide, was devised and put to practical use by MTI Inc. or Fusion Inc. so as to follow the shift of the maximum point in the electric field on generating plasma and generate plasma easily.
In each structure, a dielectric tube occupies just some area linking one central part of the waveguide wall perpendicular to the microwave electric field direction, or the H plane wall, to the counterpart of wall in the cross section of the waveguide. Each side of the area is vacant.
Therefore, it is difficult to have whole microwave absorbed in such structures without passing through both sides of the dielectric tube.
Gas certainly passes through the maximum point of the electric field, if a microwave is in H01 transmission mode. The dielectric tube, however, does not always cover the maximum point, if the point is not around the center of the H-plane wall, for example, in H02 mode. Therefore, if a rectangular waveguide is used in the apparatus, the microwave has to be tuned into the H01 transmission mode. The design of the apparatus is severely restricted due to the reason.
Furthermore, after plasma is generated in the cylindrical dielectric tube, the condition is equivalent to the one in the situation that a metallic rod is inserted into the waveguide. Then, the transmission mode in it changes and reflected wave is generated.
Therefore, a solid circuit such as a matching device or an isolator has to be provided between the magnetron microwave oscillator and the load. That results in not only the restriction in designing an apparatus but also lowering of efficiency as each functional parts are connected in the solid circuit, and the electric transmission has a loss.
A microwave plasma apparatus, which is devised to be provided a microwave transmission window in it parallel to the E plane wall, on the E plane wall of the waveguide or the extension plane of the wall so that plasma can absorb whole microwave passing through the waveguide, was disclosed as an apparatus overcoming the disadvantage in Japanese Patent Publication No. 5-3732.
In the disclosure, however, a dielectric material for the microwave-transmission window covers most of the wall, or the H plane wall, normal to the microwave electric field direction and a reflection from the plasma-generating part surrounded by dielectric members itself increases with the plasma-generating part projecting into the waveguide. If the plasma-generating part projects out of the waveguide wall, the height, or the length vertical to the H plane, of the plasma-generating part is required to be possibly decreased to maintain the transmission mode in the waveguide. Therefore, the structure also restricts the design of an apparatus severely and is inferior for the cost of production. An apparatus, which has a plasma-generating part formed unreflective-termination-shaped in order to solve the problem of the electric field change on generating plasma and maintain the plasma efficiently thereafter, was disclosed in Japanese Non-examined Patent Publication No. 7-220896 and 7-220897. In the structure, however, a disturbance of the electric field in the waveguide rises before a plasma discharge. Plasma is not easily generated with excitation by a microwave, depending on gas component for requested plasma because of its impedance, and actually reflected wave is generated, though the plasma generating part in the dielectric tube can be unreflective-termination-shaped after a plasma discharge.
Thus, the standing wave affects the magnetron oscillator while plasma is maintained. Practically, at least, an isolator has to be provided between the microwave oscillator and the load to protect the magnetron. The installation induces the restriction of cost and design on the apparatus, and lowers efficiency by transmission loss. Furthermore, in the structure, electric-discharge particles are heated to a high temperature by plasma radiation heat, and the heating prompts recombination reaction of radicals dissociated by plasma for a treatment. Consequently, the structure has a disadvantage of lowering dissociation efficiency.
Accordingly, an object of the invention is to eliminate the above disadvantages of the existing microwave discharge apparatuses. In other words, the object is to solve the problem such as the change of the electric field and the generation of a standing wave on generating plasma and maintain the plasma efficiently thereafter.
It is another object of the present invention is to produce apparatuses with abilities not only to induce plasma dissociation efficiently for the applied microwave power but generate and maintain requested particles efficiently by cooling the walls of the discharge vessel.
It is another object of the present invention is to produce a smaller apparatus costlessly.
In the invention, a microwave plasma-generating vacuum vessel, in which plasma is generated with excitation by a microwave with a wavelength of xcex under low pressure, is a dielectric tube provided in a waveguide. The shape of opening of the tube on cross sectional plane perpendicular to the direction which the dielectric tube extends to is non-circular or polygonal, at least, in a section with a length of L. Z axis is parallel to a tangent, which is drawn along the direction which the dielectric tube extends to and includes any point P in the section L. X axis is parallel to one of continuous segments which traverse the cross section of the dielectric tube normal to Z axis at the point P (not traverse the tube wall) with the maximum length of d. Y axis is normal to X axis. M axis is located along the microwave progressive direction. Whatever continuous section with a length of xcex/4 is selected , M axis is parallel to the ZX plane, at least, in a part of the section and form an angle of xcex8xe2x89xa0(2n+1)/2xcfx80 (n is an integer) radian at least at a point in the whole section. The projection of L on M axis (cosine of a segment L0 drawn from an end to the other of the section L against M axis) is not less than xcex/4 in length.
The generated active particles become inactive with their recombinations caused by being heated the discharge part of the dielectric tube due to absorbing energy. The unstable impression and the energy loss at an early stage of plasma generation are caused by a disturbance of the electric field in the metallic flame caused by the dielectric tube until the beginning of plasma discharge.
A metallic member for reflecting a microwave with a cooling function is inserted and located on the opposite side of the dielectric tube to the source of an incident microwave (the microwave oscillator) in the structure so as to solve the above problems.
The surface of the provided metallic member form an angle of xcex8xe2x89xa0(2n+1)/2xcfx80 (n is an integer) radian with M axis, at least, in a part of the surface. The dielectric tube is so located as to make a part of the tube parallel and close to apart of the metal surface. Wherein, Z axis forms an angle of xcex8xe2x89xa0(2n+1)/2xcfx80 (n is an integer) radian with M axis in the part of the tube. The projection of the segment L on M axis (cosine of the segment L0 drawn from an end to the other of the section L against M axis) is not less than xcex/4 in length. On the other hand, the part of the metal surface forms an angle of xcex8xe2x89xa0(2n+1)/2xcfx80 (n is an integer) radian with M axis. The projection of the part on M axis is not less than xcex/4. Because of the above structure, generation of reflected waves is lowered and the problems are solved.
A surface of the metallic member is formed on the opposite side of the dielectric tube to the direction in which a microwave travels. Thus, a re-entrant cavity is formed by the surface of the metallic member and the waveguide wall. The cross section of the cavity monotonously decreases along the microwave-traveling direction to the closed end. A microwave converges with traveling in the cavity and is efficiently absorbed by the dielectric discharge tube formed unreflective-termination-shaped and provided on side of the metallic member to the direction in which a microwave travels.
Same results are obtained by a system described below. A metallic member with a surface shaped to fit for the surface of the discharge tube is provided on the opposite side to the direction in which a microwave travels. Otherwise, the tube surface opposite to the direction in which a microwave travels is coated by metal or attached by a metallic plate or foil.
The metallic member, plate, foil or coat is cooled by gas or liquid. The metal cools the dielectric discharge tube.
The dielectric discharge tube combined with the metal is double-structured or holds a coolant conduit in it only on the face opposite to the microwave source direction so that the liquid can directly cool the dielectric discharge tube. Thus, gas is dissociated into plasma with absorbing microwave on the microwave-source side in the discharge tube. The tube is directly cooled by coolant on the opposite side to the microwave source.
The coolant conduit is sandwiched between the metal and the discharge part in the structure and shaped same as the unreflective-termination-shaped metal surface. Therefore, the conduit operates as an effective unreflective resistance and helps lower generation of the reflected wave.
The microwave-traveling side of the dielectric discharge tube is heated by absorbing microwave and heat radiation after the plasma generation. The gas introducing port is formed on a waveguide wall to cool the surface with coolant gas. A part of the waveguide including the discharge tube is cooled by the surroundings because the waveguide is connected with the cooling block such as metallic member.